fix(worker): block approve of a planning parent with an unfinished child, surface git stderr on ff failure

src/ClaudeDo.Worker/Runner/UnitBranchService.cs

@@ -88,8 +88,9 @@ public sealed class UnitBranchService
         var (code, stderr) = await _git.MergeFastForwardOnlyAsync(parentWt.Path, childWt.BranchName, ct);
         if (code != 0)
             return new AbsorbResult(true,
-                $"Unit branch '{parentWt.BranchName}' cannot be fast-forwarded to '{childWt.BranchName}' — the child " +
-                $"did not start from the unit's current head (e.g. it was run in parallel via Run now). git: {stderr}");
+                $"Unit branch '{parentWt.BranchName}' cannot be fast-forwarded to '{childWt.BranchName}': {stderr.Trim()} " +
+                "(usually the child did not start from the unit's current head, e.g. run in parallel via Run now, " +
+                "or the unit worktree has local changes)");
 
         var head = await _git.RevParseHeadAsync(parentWt.Path, ct);
         var diffStat = await _git.DiffStatAsync(parentWt.Path, parentWt.BaseCommit, head, ct: ct);